Crystallization is commonly employed in the chemical process industries either as means of removing dissolved impurities from a solution or as a means of recovering a desired product from solution. The removal or recovery of dissolved materials from solution generally involves either an evaporation or a cooling process. In the process of crystallization by means of cooling, the solution is cooled to lower the solubility of the dissolved material and cause it to precipitate or crystallize from the solution. Suitable crystallization apparatus commonly includes a heat exchange unit. Typically, such a unit comprises a tank, holding the solution to be cooled, and a means, such as a tube or a series of tubes, for example in the form of a helical coil, through which is circulated a liquid cooling medium. As the temperature of solution is lowered, solubility of the dissolved materials is decreased and precipitation or crystallization results. Agitators are often required to maintain the crystallizing material in suspension until it is transferred from the unit, for example, to a holding tank. Depending on the degree of crystallization achieved, the solution may have to be recycled through the cooling tank a number of times. Apparatus employed for such purposes is frequently complex, often involving a number of separate tanks and recycling systems, and large, especially in terms of floor space required. In addition, such systems often involve a relatively low rate of circulation of fluids, in order to permit a sufficient residence time in the heat exchange unit for the desired cooling to take place. Low circulation rates of a crystallizing fluid often result in the collection of crystallized particles as a coating on the solid surfaces of the apparatus and a resultant fouling of the apparatus. As a result, not only have the manufacturing costs of such units been high, but installation, operation, and maintenance costs as well.
Accordingly, it is an object of the present invention to provide an improved cooling crystallization apparatus of an integral forced circulation type, which is especially useful for the cooling of caustic catholyte liquor from electrolytic diaphragm cells and the crystallization of dissolved salts therefrom.
It is a further object of the present invention to provide an improved, forced circulation cooling crystallizer having a relatively compact, simple design having improved maintenance characteristics and which will operate efficiently and economically over a wide range of operating conditions.
These and other objects and advantages will become apparent to those skilled in the art from the following detailed description.